The world's population is growing at a rate of a little more than 1% per year.
However, not all segments of the population are growing at the same rate.
This graph shows the rate of growth of 3 different groups: children, adults, and the elderly.
As you can see, while the population of children is fairly constant, the elderly population is increasing at the fastest rate.
This is due to very low birth rates in developed countries and birth rate declines in most developing countries.
As a result of these trends, the world's population is aging.
Between 2015 and 2050, the percentage of world's population over 60 is expected to nearly double.
In 2015 there were 900 million people over 60; in 2050 the number should be around 2 billion.
Aging populations are becoming a huge problem in many countries, especially developing countries.
It used to be that many elderly people lived with their family and helped take care of the next generation.
They had a place to stay and people to help take care of them when they needed assistance.
Now however, many elderly people, or senior citizens, are left on their own.
This is partly the result of people having fewer children and also because of massive urbanization.（大规模的城市化）
Young and middle-aged adults have left the rural areas for economic opportunities only available in large urban centers.
As a result, there is no one to care for the elderly who are left behind to live on their own.
This is especially true in developing countries.
With an aging population, health problems become a growing concern.(健康问题日益收到关注）
Some of the leading conditions that accompany aging include vision problems, hearing loss, diabetes(糖尿病）, and dementias（痴呆） such as Alzheimer's disease.
These problems can lead to disabilities that make it difficult to live without assistance.
With diseases such as Alzheimer's, people lose their memory and are unable to recognize their own children.
Worldwide, more than 46% of people aged 60 and over have disabilities.
Getting meals and medical care, for example, becomes both expensive and difficult.
As a result, the costs of providing services to the sick and disabled can become a huge problem.